Revealing mistake: When the Christians are being fed to the lions, we can see that when a lion "attacks" one of the men, the man is actually smiling and visibly playing with that wild animal.
Other mistake: When one of the Christians is being crucified and burnt, we can see that just after the man's "death" the body is substituted by a plastic doll covered in red paint.
Deliberate mistake: Wooden crosses are shown in several scenes, hanging on a wall. Crosses did not come into use as a Christian symbol until 450 A.D. (about 400 years after the film is set).
Visible crew/equipment: An absolute cracker from an epic movie - in one of the huge crowd scenes when they all go through the huge gates, you can clearly see a wee man in blue overalls eating a sandwich, he quickly realises there is a shoot on and tries to blend in with the crowd and enters through the gates.